This proposal plans to examine the cellular mechanism of acid excretion by the kidney. We use the turtle urinary bladder as a model for H ion transport in the renal tubule. The H ion pump is located in the luminal border of this epithelium and depends for its function on metabolic energy. The major area of effort in this proposal is to identify the source of metabolic energy. We found that the H ion pump is a very tightly coupled system with a high efficiency of energy conversion. This implies that it is reversible. We were able to produce reversal of this pump and found that it resulted in ATP synthesis. Hence we conclude that the pump is a reversible ATPase. To estimate the H/ATP stoichiometry we measured the free energy of ATP hydrolysis in the cell and found it to be approximately to kJ/mole, and we also measured the reversal potential of the pump which was approximately 180 mV. The ratio of the two forces should give us the stoichiometry and we found it to be approximately 3.0 H/ATP. Our future plans include attempts at isolation of the H ion pump from the luminal membrane of the turtle bladder.